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    SubjectRe: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention
    On 12/16, Peter Zijlstra wrote:
    >
    > +static int ttwu_force(struct task_struct *p, int wake_flags)
    > +{
    > + struct rq *rq;
    > + int ret = 0;
    > +
    > + /*
    > + * Since we've already set TASK_WAKING this task's CPU cannot
    > + * change from under us.

    I think it can. Yes, we've set TASK_WAKING. But, at least the task
    itself can change its state back to TASK_RUNNING without calling
    schedule. Say, __wait_event()-like code.

    > +static int
    > +try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
    > {
    > - int cpu, orig_cpu, this_cpu, success = 0;
    > + int cpu = task_cpu(p);
    > unsigned long flags;
    > - unsigned long en_flags = ENQUEUE_WAKEUP;
    > - struct rq *rq;
    > + int success = 0;
    > + int load;
    >
    > - this_cpu = get_cpu();
    > -
    > - smp_wmb();
    > - rq = task_rq_lock(p, &flags);
    > - if (!(p->state & state))
    > - goto out;
    > + local_irq_save(flags);
    > + for (;;) {
    > + unsigned int task_state = p->state;
    >
    > - cpu = task_cpu(p);
    > + if (!(task_state & state))
    > + goto out;

    Well, this surely breaks the code like

    CONDITION = true;
    wake_up_process(p);

    At least we need mb() before we check task_state the first time.

    Oleg.



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